The work in progress under this proposal concentrates on the requirements of the cell for DNA replication. We are investigating the mechanism of DNA replication in E. coli cells which appear to have the ability to bypass a defect in DNA polymerase III, yet continue to replicate DNA. Such cells in some instances appear to have a novel suppression product from the polA gene and in some instances to have a wild type polA gene product. It appears that such strains carry a mutation outside of any of the DNA polymerase loci which enables them to replicate their DNA in the presence of a functional DNA polymerase I activity. For the coming term of the grant, our studies shall focus upon the enzymatic and physical characteristics of the unusual DNA polymerase activity which is seen in cases of suppression. In addition, we shall conduct genetic and biochemical studies designed to define the location and character of the mutation which permits DNA polymerase I to apparently be the sole support of DNA replication in such cells. These studies shall make use of classical genetic mapping techniques and in vitro DNA replication systems.